(a) Field of the Invention
The present invention relates to a negative active material for lithium rechargeable batteries and a method of fabricating the same, and particularly to a negative active material in which the swelling phenomenon is suppressed and cycle-life characteristics are improved, and a method of fabricating the same.
(b) Description of the Related Art
A rechargeable lithium battery having an average discharge voltage of approximately 3.7 V is considered to be one of the essential components in the digital generation since it is an indispensable energy source for portable digital devices, such as cellular pones, notebook computers, and camcorders (i.e., the so called “3C” devices).
Currently, more than 95% of the negative active material used for lithium rechargeable batteries is made of graphite having a high crystallinity. Graphite is generally classified into artificial graphite, in which the particle is processed, and natural graphite, in which the particle is not processed, and it has a characteristic shape, particle size, and surface features depending upon the source and the processing method. Accordingly, graphite is considered as a main factor in determining both the efficiency at early cycles and the long-term cycle-life characteristics of a lithium rechargeable battery.
For a rectangular-type battery, the thickness of the battery varies depending upon the type of graphite since the graphite undergoes a side reaction with electrolytes resulting in the production of gas. In detail, the electrolyte may undergo a side reaction with the graphite at the surface thereof in the early battery charge stage so that the surface of the graphite will be coated with a thin layer of a lithium organic compound, called a Solid Electrolyte Interface (SEI). In this side reaction, gas is also produced, and it makes the battery expand, which is called a swelling phenomenon and which causes problems to users. The phenomenon is determined according to the types of electrolyte and graphite used in the battery.
The cycle-life characteristics of the lithium rechargeable battery are dependent upon the type of SEI layer formed on the graphite. A more serious swelling problem occurs on the electrode plate during repeated cycles of several tens or hundreds. It is postulated that the swelling phenomenon results from the electrochemical reaction between the surface of the negative active material and the electrolyte; and in countries having a climate of high temperature and high humidity, the swelling phenomenon tends to occur more often when the battery is operated under high temperature. In this case, it is reported that the negative active material is also considered to be a decisive factor for the phenomenon.
To meet world demands for a battery having a high capacity, battery manufacturers have put a great deal of effort into solving the swelling problem. One proposal is to modify the surface of the graphite so that the efficiency during early cycles is improved, the gas generated from the side reaction with electrolytes is prevented, and the cycle-life characteristics are thereby improved. Another is to provide a novel functional electrolyte.
It has also been suggested that a negative active material be coated with graphite or amorphous carbon to maintain stability during reaction with electrolyte, that an additive such as vinyl carbonate or vinyl sulfone be added to the negative active material to increase the stability of the surface of the negative active material, and that the negative active material be treated with a metal alkoxide solution. However, since research has not yet solved the swelling problems to a satisfactory extent, demands to solve the problem continue.